Conventionally, comparatively soft polishing pads obtained by impregnating an unwoven fabric with a polyurethane resin and hard polishing pads made of foamed polyurethane have been used for polishing pads for processing a mirror surface of a semiconductor wafer used as a substrate for forming an integrated circuit or processing unevenness of an insulating film or an electric conductor film into flatness during semiconductor device production (For example, see patent documents 1 to 3).
In particular, in order to have both high flatness in a local scale (die size) and evenness of a film thickness in a global scale (wafer size) accompanied by miniaturization and multilayer structures of recent semiconductors, a double layered pad having a hard polishing layer as a surface layer and a soft cushion layer on the rear surface has been commonly used. In such a double layered pad, the cushion layer on the rear surface deforms the polishing layer as the surface layer conforming with undulation having a comparatively long wavelength of a film to be polished present in the wafer surface and thus serves as making the whole wafer uniformly polished. Layers having a specific strain coefficient, a volume elastic modulus, and the like have been suggested as a cushion layer used in the double-layered pad (For example, see patent documents 4 to 6).
In recent years, miniaturization, high integration, and multilayered wiring of semiconductor devices have been furthermore progressed, and a polishing pad has been also required to stably achieve further higher flatness and evenness of a film thickness after polishing. However, in the polishing pad shown in patent document 4, there is no indication as the characteristics required for a cushion layer, other than a strain constant when compression pressure is received, and the strain constant shown herein easily causes decrease in a polishing rate due to a too soft cushion layer. Further, a deformation amount measured in a state where a compression pressure is constant or monotonously increases (or monotonously decreases) does not correspond to a deformation amount in a case where a compression pressure is repeatedly changed in a very short time cycle such as a time of actual polishing in many cases.
On the other hand, the polishing pad shown in patent document 5 or 6 does not necessarily have sufficient conformity to undulation of a film on a wafer surface; as a result, evenness of a film thickness after polishing may be insufficient. When particularly a polishing layer as a surface layer is made from a resin with a non-foam structure, such a polishing layer has high heat conductivity as compared to a polishing layer with a foam structure, and thus, temperature increase due to heat generation during polishing easily transmits to a cushion layer on the rear surface to change physical properties of the cushion layer due to temperature increase, which easily gives an influence on polishing characteristics such as uniformity. In particular, a material to be polished has a wiring metallic film and the metallic film is formed from copper and the like, heat generation during polishing is large and the above described influence is likely to appear.
A polishing pad obtained by using a cushion layer having a micro rubber A hardness of 50 or more and 90 or less, and a specific hysteresis loss and a tan δ value has been known (see patent document 7). However, the polishing pad shown in patent document 7 does not necessarily have an appropriate deformation amount of the cushion layer for undulation of a film on a wafer surface, and when a deformation amount of the cushion layer is too small, evenness of a film thickness after polishing becomes insufficient, and on the other hand, when a deformation amount of the cushion layer is too large, in addition that a polishing rate easily decreases, there is a problem that evenness of a film thickness after polishing also decreases. In the same manner as the polishing pads described in patent documents 5 and 6, particularly when a polishing layer as a surface layer is formed from a resin with a non-foam structure, such a polishing layer has high heat conductivity as compared to a polishing layer with a foam structure, and thus, temperature increase due to heat generation during polishing easily transmits to a cushion layer on the rear surface to change physical properties of the cushion layer due to temperature increase, which easily gives an influence on polishing characteristics such as uniformity. In particular, a material to be polished has a wiring metallic film and the metallic film is formed from copper, and the like, heat generation during polishing is large and the above described influence is likely to appear.    patent document 1: JP-A-5-8178    patent document 2: JP-A-2000-178374    patent document 3: JP-A-2001-89548    patent document 4: JP-A-5-505769    patent document 5: JP-A-2000-117619    patent document 6: JP-A-2000-202763    patent document 7: JP-A-2006-339570